Here's an example of how you would make an HTTP POST call to a web service using JQuery: http://api.jquery.com/jQuery.post/
I am going to assume you are calling a soap service, so I used a call to the mashups API, which is why the destinationURI is ..\ and not a full URL.
NOTE: the last line of xmlDOC = ... is the return response from the SOAP service. Also note this is being sent synchronously, IE, will wait for a return from the service. If this is a service that takes a long time, then you might write a return function and pass true with the POST command to make it asynch.
Hope this helps. (had to make the code an image. Was having issues formatting the XML, and done have time to fool around with it right now.)